Notes:

The serious problem with the password approach, whether used with telnet or with ssh, is that the password you need to enter at the client end is stored on the server. Even though it’s stored in an encoded form in /etc/passwd or /etc/shadow, this password can be cracked with brute force once one has access to that file. The difference with the public/private key split is that if an attacker gets the public key stored on the server, that public key cannot be used to get back into the server! Only the private key, kept on the client only, can be used to get into a server with the public key.